There are extensive publications relating to the use of xanthan heteropolysaccharide biopolymers as viscosifying agents for particle transport fluids such as drilling muds, fracturing fluids, or filter structure emplacement fluids. Early work on xanthan polymers was done at the Northern Regional Research Laboratory of the United States Department of Agriculture at Peoria, Ill. By way of exemplification U.S. Pat. No. 3,198,268 discloses xanthan fermentates being employed in drilling muds and for other oil production uses in 1965. By way of further exemplification, U.S. Pat. No. 3,251,417 discloses xanthan polymers being employed with foaming agents for air drilling.
The xanthan heteropolysaccharide biopolymers produced by Xanthomonas campestris and which are widely used in drilling muds and for viscosifying agents in other oil field applications, have been characterized as having the following formula shown in Haworth convention: ##STR1##
Chemically, the wild or native xanthan polymer is an anionic heteropolysaccharide. The repeating unit of the polymer is a pentamer composed of five sugar moieties, specifically two glucose moieties in the repeating chain unit, two mannose moieties on the side-chains, and a glucuronic acid moiety at the end of the mannose sidechains. Usually, this basic structure is specifically acetylated and pyruvylated as shown and as described for example by Janson, P. E., Kenne, L., and Lindberg, B., in Carbohydrate Research, 45:275-282 (1975) and Melton, L. D., Minot, L., Rees, D. A., and Sanderson, G. R., in Carbohydrate Research, 46:245-257 (1976). The extent of acetylation and pyruvylation is known to vary.
In the formula shown, n can be an integer such that the molecular weight is over 1 million, and M.sup.+ is a hydrogen ion or an alkali metal ion such as a sodium ion or a potassium ion.
Xanthan gums find great utility in particle transport fluids such as in drilling muds because a very low concentration, say about 1% solution, imparts pseudoplastic viscosity and keeps mineral particles in suspension. In pseudoplastic systems, the viscosity decreases as the shear rate is increased. This is an instantaneous completely reversible process. Milas, M. et al., Polymer Bulletin 14:157-164 (1985) disclose viscosity dependence of xanthan polymers as a function of polymer concentration, shear rate, and molecular weight.
Reference is also made to U.S. patent application Ser. No. 07/696,732 filed May 7, 1991, now abandoned, by Doherty et at. of Synergen, Inc. entitled "Genetic Control of Acetylation and Pyruvylation of Xanthan Based Polysaccharide Polymers", published 12 Nov. 1992 from International Application Number PCT/US92/03448 and designated International Publication Number WO 92/19753. That reference is specifically referred to and incorporated herein by reference. In particular, the following material is quoted:
"The present invention discloses a family of xanthan based polysaccharides having improved properties relative to naturally-occurring xanthan gum. Modifications of xanthan gum have been previously described. For example, Bradshaw et al. (Carbohydrate Polymers, 3:23-28 (1983) describe methods for preparing chemically modified xanthan gum which is deacetylated or de-pyruvylated. Various means of chemically deacetylating xanthan gum produced by Xanthomonas campestris also are described in U.S. Pat. Nos. 3,000,790 and 3,054,689. To date, the predominant method utilized for these deacetylation processes has been chemical removal of the acetate moieties from normally acetylated xanthan gram. It has been found that chemical processes for deacetylating xanthan gums can result in a number of undesirable side effects and may cause hydrolysis of the glycosidic backbone, resulting in an irreversible change in the conformation of the molecule and lowered molecular weight. PA1 Some of the rheological properties of deacetylated xanthan in aqueous media are known. See, e.g., Tako and Nakamura, Agric. Biol. Chem.48:2987-2993 (1984) and U.S. Pat. Nos. 3,000,790 and 3,054,689. Also, a method of increasing the viscosity of an aqueous solution using a deacetylated polysaccharide is described in U.S. Pat. No. 3,096,293. Thus, a method for obtaining non-acetylated xanthan which does not cause untoward side effects has been sought. PA1 Xanthan gum can be chemically de-pyruvylated as well, as described by Holzwarth and Ogletree in Carbo. Res. 76:277-280 (1979). This chemical method of de-pyruvylation also can alter the xanthan polymeric unit and/or cause hydrolysis of the glycosidic backbone. While a strain of X. campestris has been described in U.S. Pat. No. 4,296,203 which produces non-pyruvylated xanthan gum, this non-pyruvylated gum was either fully acetylated or deacetylated using chemical means. PA1 Additionally, the extent of acetylation of the internal mannose on the xanthan side chain and the extent of the pyruvylation of the terminal mannose may vary. The present inventors believe that a fully acetylated and/or fully pyruvylated xanthan will have improved rheological properties for certain oil recovery properties. PA1 Moreover, the present inventors have identified polysaccharides which are based on alterations of the normal xanthan pentamer building block."
Summarily, the Synergen reference discloses biosynthesis of polymers having structures as shown in FIG. 1.
A series of patents are exemplary of the state of the art relating to the use of wild or native xanthan polymers in fluids for oil patch applications. For example, U.S. Pat. No. 3,319,715 discloses the use of Mg(OH).sub.2 plus wild or native xanthan polymer as a fluid loss additive for fluids employed as drilling fluids, reworking fluids, perforating fluids, fracturing fluids, etc. U.S. Pat. No. 3,729,460 discloses improved thickening to be obtained from xanthan polymers by heating with an alkali metal hydroxide compound.
Another series of references discloses use of cross linked viscous compounds such as guar gum, locust bean gum, etc. as thickened fluids for use in oil production operations. For example, U.S. Pat. No. 4,033,415; U.S. Pat. No. 4,369,124; U.S. Pat. No. 4,534,870; U.S. Pat. No. 4,462,917; U.S. Pat. No. 4,464,270: U.S. Pat. No. 4,488,975; U.S. Pat. No. 4,798,902; U.S. Pat. No. 4,799,550; U.S. Pat. No. 4,477,360; and U.S. Pat. No. 4,514,309 disclose use of a variety of cross linking agents to prepare cross linked polysaccharides and to prepare a variety of thickened fluids employed in oil field applications.
None of the foregoing references appear to disclose or suggest use of a non-acetylated but otherwise unmodified xanthan heteropolysaccharide polymer plus guar gum to impart viscosity to an aqueous particle transport fluid sufficient to suspend mineral particles, as is disclosed and claimed according to the invention at hand, however. By "non-acetylated" xanthan we mean xanthan produced without an acetyl group by biosynthesis such as by the Synergen reference above and having the structure shown in FIG. 1. In contrast, by "deacetylated" xanthan we mean xanthan which has had its acetyl group removed by chemical or other means.
The following disclosures are of particular relevance to the invention at hand:
Dea, I. C. M. and Morris, E. R, "Synergistic Xanthan Gels", (inventor/ACS Symposium Series, 45 (1977) 174-181) disclose that native xanthan polymer does not form a gel, but on mixing with locust bean gum, another non gelling polysaccharide, firm rubbery gels are formed at low polymer levels. It is also disclosed that guar gum does not gel with xanthan polymer in any concentration and that weaker gels are obtained with gum tara. Molecular explanations for the strange phenomena are provided.
Dea, I. C. M., Morris, E. R., Rees, D. A., Welsh, E. J., Barnes, E. A., and Price, J., "Associations of Like and Unlike Polysaccharides Mechanism and Specificity in Galactomannans, Interacting Bacterial Polysaccharides, and Related Systems", Carbohydrate Research, 57 (1977) 249-272 also disclose that locust bean gum gels with native xanthan polymers and a xanthan polymer alone does not gel at any concentration. The reference also discloses that guar gum does not gel at all when mixed with xanthan polymer. Various molecular theories are proposed to explain the phenomenon.
Much the same disclosure is made by Rees, D. A. "Shapely Polysaccharides", Biochem, J. (1972) 126, 257-272.
Much the same disclosure is also made by Rocks, J. K., "Xanthan Gum" (inventor/Food Technology, 25 (1971) 476-485).
Perhaps the closest reference to the invention at hand of which the inventor is aware is that of Tako, M., Nakamura, S., "Synergistic Interaction Between Xanthan and Guar Gum", Carbohydrate Research, 138 (1985) 207-213. In particular, FIG. 3 on Page 211 plots dynamic viscoelasticity of mixtures of guar gum and native xanthan polymer and of guar gum and deacetylated xanthan polymer. FIG. 3 shows the effect at 25.degree. C. of the ratio of xanthan (native and deacetylated) to guar gum in solution on the dynamic viscoelasticity of a total gum concentration of 0.2%. In the case of the mixture with native xanthan, little synergistic increase in dynamic viscoelasticity is observed. However, the synergistic interaction was enhanced in the mixture with deacetylated xanthan, indicating the intermolecular interaction resulted from deacetylation of xanthan. FIG. 3 shows the maximum dynamic modulus was achieved when the mixing ratio of deacetylated xanthan to guar gum was 2:1.
The deacetylated xanthan polymer employed is stated to be chemically deacetylated, which means that it is otherwise modified by the chemical deacetylation procedure. Even though the results reported in the article do show a synergistic interaction in increasing high shear viscosity, the synergistic interaction is not in any way predictive of the much greater and different effect of suspending mineral particles at very low shear obtained according to the invention at hand which is disclosed and claimed in this application.
In other words, prior art fluids can be formulated with higher shear viscosities which are equivalent to the fluids of this application, but these fluids do not suspend particles any where nearly as effectively as the fluids of this application, as demonstrated by the runs disclosed in this application.